Issue 3B problem
Hi!
I have an issue 3B board that is having a strange problem: when I got it it didn't had the -5V and +12v, as usual. After replacing TR4 and TR5 the -5V came back, but the 12V are actually 11.10V... :-?
The machine seems to work, the coil buzzes but I have no image... I connected via composite video (bypassing the modulator) and all I got is some faint and moving interference with black background. LM1889 was replaced and socketed with a known working one.
Do you have any clue? :???:
I have an issue 3B board that is having a strange problem: when I got it it didn't had the -5V and +12v, as usual. After replacing TR4 and TR5 the -5V came back, but the 12V are actually 11.10V... :-?
The machine seems to work, the coil buzzes but I have no image... I connected via composite video (bypassing the modulator) and all I got is some faint and moving interference with black background. LM1889 was replaced and socketed with a known working one.
Do you have any clue? :???:
Post edited by Hernán Pablo Álvarez on
Comments
None whatsoever :roll: But i'd want to find out if it's a "black screen, nothing happening", or a "no video signal generated". The first would indicate a memory / CPU / clock signal problem, the latter would indicate a problem in ULA or video encoding circuitry.
As a first step, I'd pull out the ULA chip (since it's normally socketed), place it in another (known working! preferably same issue) Spectrum, and see if that works. And perhaps the reverse too: take the (working) ULA from that other Spectrum, and plug it in your non-working board.
At that point, it should be clear whether you have a faulty/dead ULA, or some other problem. Do you have this option, to swap ULA's with another (working) Spectrum?
That's right, I've been doing tests today and indeed, 11.5V is enough, I have a working Speccy with that voltage in the 12V line.
After finding that, I connected the video output to the negative end of C65 as told on S-Video mod on Sami Vehmaa's site (http://user.tninet.se/~vjz762w/) to see if I could get something... and I got a extremely faint, ghostly and jumpy (c)1982 Sinclair Research message!!!
I'm not a lot into electronics, but I suspect that is some problem with video output amplification...
The video modulator is out of the picture here, and you already replaced the video encoding IC. So I'd absolutely agree with your guess about likely cause.
To be honest, high frequency analog video stuff isn't my cup of tea either :( Looking at the schematic, I'd suspect TR1 or TR2 transistor (both ZTX313). Transistors like this are dirt cheap, but I wouldn't know a suitable replacement type. You could try a general purpose NPN type, but I'm not sure that would do (try a type that's used in video applications elsewhere?). ZTX313 datasheet can be found online btw.
Other possibilities, in order of easy to try / check:
For that last one: you might spot something quick, or waste a lot of time concluding the board itself is fine. I wouldn't spend that time before trying other options...